An example of a prior art power supply device that is used in electronic devices such as a liquid crystal display device is shown in FIG. 33. It should be noted at this point that the description below takes a power supply device used in a liquid crystal display device by way of example only. This power supply device 320 comprises a voltage regulation portion 322 and a multi-value voltage generation portion 324.
In this case, the voltage regulation portion 322 has the function of generating a regulated voltage Vreg by adjusting a voltage between two supply voltages VS and VDD, and comprises a control portion 314 and a voltage-divider resistor 313. The control portion 314 further comprises switches S1 to S4 that control the resistance of the voltage-divider resistor 313 on the basis of regulated-voltage setting signals that are input thereto. The voltage-divider resistor 313 comprises resistors R1 to R4, these resistors R1 to R4 are selectively bypassed by control from the control portion 314, this varies the resistance of the voltage-divider resistor 313, and thus the regulated voltage Vreg is determined. Enabling voltage regulation in this manner allows the user to adjust the contrast of the liquid crystal display.
The multi-value voltage generation portion 324 further comprises a voltage-divider resistor 312 formed of resistors Ra to Re. It has the function of dividing the regulated voltage Vreg from the voltage regulation portion 322 to generate supply voltages V0 to V5 of different magnitudes, This generation of multi-value supply voltages V0 to V5 makes it possible to implement a method such as a 6-level drive method to drive the liquid crystal display.
Another example of a prior art power supply device is shown in FIG. 34. The power supply device 321 of this figure differs from that of FIG. 33 in that a multi-value voltage generation portion 326 thereof comprises operational amplifiers (OP-amps) 301 to 305 connected in a voltage-follower manner. Each of these OP-amps 301 to 305 is connected to one of divider terminals (taps) 330 to 338 of the voltage-divider resistor 312. These OP-amps 301 to 305 convert the impedances of the divided voltages generated at the corresponding divider terminals 330 to 338. In this prior art power supply device, all of the OP-amps 301 to 305 have the configuration that will be described later with reference to FIG. 10 (n-type OP-amp).
The voltage regulation portion 322 shown in FIG. 33 and FIG. 34 turn on and off switches S1 to $4 of the control portion 314, on the basis of the regulated-voltage setting signals. This adjusts the number of steps in the voltage-divider resistor connected between the supply voltages VS and VDD, to generate the regulated voltage Vreg. This regulated voltage Vreg is then divided by the voltage-divider resistor 312 of the multi-value voltage generation portion 324 or 326. In the configuration of FIG. 33, these divided voltages are output without any impedance conversion as the multi-value driving supply voltages V0 to V5. On the other hand, in the configuration of FIG. 34, the impedances of these divided voltages are converted by the OP-amps 301 to 305 that are connected in a voltage-follower manner, to generate the multi-value driving supply voltages V0 to V5 that are output.
These driving supply voltages V0 to V5 are supplied to a liquid crystal drive signal generation portion (LCD driver) that is not shown in the figures. This drive signal generation portion generates drive signals for driving the liquid crystal panel on the basis of these driving supply voltages V0 to V5.
Liquid crystal display devices are often used in portable electronics equipment. That is why it is considered that the demand current of such a liquid crystal display device must be made extremely low to reduce the power consumption. A further concern is not only to reduce the power consumption of the liquid crystal display device in this manner, but also increase its display quality. In order to ensure a lower power consumption for the liquid crystal display device, it is necessary to reduce the power consumption of a power supply device that supplies power to the liquid crystal display device. Further, in order to ensure a higher display quality for the liquid crystal display device, the supply voltages supplied from the power supply device must be such that they do not adversely affect the display quality of the liquid crystal display device.
In view of the above concerns, the prior art power supply devices 320 and 321 of FIG. 33 and FIG. 34 have the problems described below.
To enable the regulation of the contrast of the liquid crystal display as described above, the power supply device of the liquid crystal display device enables voltage regulation. In the prior art examples shown in FIG. 33 and FIG. 34, this voltage regulation is provided by the voltage regulation portion 322 varying the number of divisions in the resistors connected between the supply voltages. Assume that the resistances of the voltage-divider resistors 312 and 313 are R12 and R13. Thus the resistance R12 is fixed at R12=Ra+Rb+Rc+Rd+Re. The resistance R13 is determined by which of the switches in the control portion 314 are on. For example, if the ratios of resistances R4 to R1 are set to 8:4:2:1, and switches S4 to S2 are turned off with S1 on, R13=R4+R3+R2=14R (where the resistance of R1 is assumed to be R). In this manner, the resistance R13 can be varied in steps from, for example, 0 to 15R (=R13tot) by turning the switches S4 to S1 on or off by the regulated-voltage setting signals.
In these prior art power supply devices, the regulated voltage Vreg is determined by the ratio of these resistances R12 and R13, as expressed by the equation below. Note that in the description below, VDD is assumed to be 0 V and VS is assumed to be a negative voltage such as -9 V. EQU Vreg=VS.multidot.R12/(R12+R13) Equation 1
In this case, the resistance R13 can be varied between 0 and 15R (R13tot), as described above, so that the value of Vreg can be varied as shown in FIG. 35A. For example, if R13=0 (S4 to S1 are all on), Vreg has a maximum value Vrmax (negative) given by the following equation: EQU Vrmax=VS Equation 2
Further, if R13=R13tot=15R (S4 to S1 are all off), Vreg has a minimum value Vrmin (negative) given by the following equation: EQU Vrmin=VS.multidot.R12/(R12+R13tot) Equation 3
Therefore, a voltage regulation range Vrange is given by the following equation: EQU Vrange=.vertline.Vrmax-Vrmin.vertline.=.vertline.VS.vertline..multidot.R13t ot/(R12+R13tot) Equation 4
Since it is desired that a power supply device used in a liquid crystal display device should provide a wide range of contrast regulation, the voltage regulation range Vrange should also be capable of being set to as wide a range as possible. As can be understood from Equation 4, if it is desired to widen the voltage regulation range Vrange of either of the above prior art examples, it is necessary to either reduce the resistance R12 of the voltage-divider resistor 312 that sets the number of divisions, or increase the total resistance R13tot of the voltage-divider resistor 313 that enables the switching of the steps. However, with the former method, since the resistance of the voltage-divider resistor is small, the consumption of current flowing between the supply voltage VDD and the supply voltage VS is large, the problem of providing a low power consumption cannot be solved. With the latter method, when this circuitry is mounted in a semiconductor integrated circuit, the aspect ratio of the resistors made of a material such as polysilicon becomes too large, causing the problem that the chip area increases.
Further, when voltage regulation is provided by a power supply device of this type, it is necessary to set a central value Vc for performing the voltage regulation. This central value Vc becomes the value at the center of the contrast brightness range when the contrast of the liquid crystal display is adjusted. It is desirable to set the central value Vc to, for example, S4 to S1=(0111) (where 0 means off and 1 means on) as shown in FIG. 35A. This enables voltage regulation within a range of, for example, seven levels above and eight levels below the central value, so that contrast regulation can be provided over a range that is the same on both the light side and the dark side. However, manufacturing variations occur in the semiconductor devices or liquid crystal display elements that include this power supply device, due to factors such as changes in processing conditions. If such variations occur, there will also be variations in the central value Vc of brightness for contrast regulation. In such a case, the maximum value, minimum value, and voltage regulation range of the regulated voltage in the prior art power supply device are fixed by the resistances R12 and R13 of the voltage-divider resistor, as is clear from Equations 1 to 4. Therefore, if a change in the central value Vc should be caused by manufacturing variations in this manner, it is not possible to shift the maximum value, minimum value, and voltage regulation range upward or downward. Thus, if, for example, the central value Vc shifts to a value set by S4 to S1=(0100), as shown in FIG. 35B, voltage regulation in the range above Vc can be performed over only four levels, and it is no longer possible to provide contrast regulation over ranges that are the same on both the light side and the dark side of the center. This makes it impossible to solve the problem concerning improving the display quality. One method of solving this problem that has been considered is to increase the number of divisions of the voltage-divider resistor 313 and thus broaden the range of voltage regulation, to allow for manufacturing variations, but this method causes a further problem in that it increases the area of the semiconductor chip. Further, since voltage regulation in the prior art power supply device is provided by switching the number of divisions of the voltage-divider resistor, it is necessary to store the value for determining this central value Vc, such as the value (0111) in FIG. 35A and (0100) in FIG. 35B, in means such as non-volatile memory, which raises the problem of making the circuit configuration complicated when it comes to building the system.
In the prior art examples shown in FIG. 33 and FIG. 34, it is clear from Equation 1 that the regulated voltage Vreg is determined by the supply voltage VS or the like, and the resistance ratio of the voltage-divider resistors 312 and 313. Therefore, there is a problem in that, if the supply voltage should change, the regulated voltage Vreg will also change, such that in a liquid crystal display device that uses a battery as a power source, any change in the voltage of the battery will lead to a change in display quality.
Now consider the multi-value voltage generation portions 324 and 326 of FIG. 33 and FIG. 34.
In general, in a time-division (multiplexed) system for driving liquid crystal, six supply voltages obtained by calculation by a known 6-level drive method (method of voltage averaging, amplitude selective addressing scheme) are used. These voltages are called V0, V1, V2, V3, V4, and V5, starting from the highest. A liquid crystal display device has common electrodes and segment electrodes, where the common electrodes are provided with a common signal (scanning signal) for determining whether or not lines are selected. Further, the segment electrodes are provided with a segment signal (data signal) for determining whether or not display pixels are lit. The voltage of each common electrode is V5 (or V0) in a selected period or V1 (or V4) in a non-selected period. When the voltage of a common electrode is V5 (or V0), if the voltage of a segment electrode is V0 (or V5) the corresponding pixel is lit; if that voltage is V2 (or V3), the corresponding pixel is not lit. Note that the values in parentheses in this case indicate the supply voltages when the polarity of a frame (FR) signal is inverted. This frame signal is an alternation signal in frame inversion technique or line inversion technique.
These multi-value supply voltages V0 to V5 are generated by either of the multi-value voltage generation portions 324 and 326. In this case, the multi-value voltage generation portion 324 of FIG. 33 divides the supply voltage by the voltage-divider resistor 312 and the resultant values are used unchanged as V0 to V5. However, from consideration of display quality and power consumption reduction, it is not preferable to use these resistance-divided voltages unchanged as supply voltages for liquid crystal drive. In other words, to ensure a lower power consumption for the device, the resistances of the resistors Ra to Re that form the voltage-divider resistor 312 must be as high as possible, so that the currents flowing through the voltage-divider resistor 312 are as low as possible. However, if the resistances of Ra to Re are made high, the output impedances at the divider terminals 330 to 338 of the voltage-divider resistor 312 will also be high. If the output impedances are made high in this manner, changes in the supply voltages during the liquid crystal drive will be great, adversely affecting the liquid crystal display quality. Therefore, this method of generating multi-value supply voltages is not suitable for driving a large liquid crystal panel.
On the other hand, in the method shown in FIG. 34, the above problem is solved by using the OP-amps 301 to 305, which are connected in a voltage-follower manner, to convert the impedances of the divided voltages generated at the divider terminals 330 to 338. In other words, the output impedances of the multi-value voltage generation portion 326 are reduced by the impedance conversion provided by the OP-amps 301 to 305, so that deterioration of the liquid crystal display quality can be prevented. When such impedance conversion is provided, increasing the output impedances at the divider terminals 330 to 338 causes no problems, so the resistances of Ra to Re can be increased. If the resistances of Ra to Re are increased, the currents flowing through the voltage-divider resistor 312 can be reduced, and thus it becomes possible to design a device with reduced power consumption.
To attempt to reduce the power consumption of the device even further, it is necessary to restrain the power consumed by the OP-amps S01 to 305 as well. Each of-these OP-amps 301 to 305, as will be described later with reference to FIG. 10, is provided with a drive portion having a resistor or constant-current source connected at one end to a high-potential power-source side and an n-channel drive transistor connected at one end to a low-potential power-source side. In order to restrain the power consumption of the OP-amps 301 to 305, it is necessary to reduce the current flowing in this drive portion (said resistor or constant-current source).
However, if the current flowing in this drive portion is reduced in order to reduce the power consumption, the problem then arises that the phenomena of shadows and cross-talk will occur in the liquid crystal display, and the liquid crystal display quality will become extremely low. With a drive method called a 6-level drive method (method of voltage averaging, amplitude selective addressing scheme), the effective voltages applied to the pixels during the drive period are averaged for all the on pixels and for all the off pixels, to try to average the display status. Therefore, an averaging (equalization) state that is a precondition of this 6-level drive method cannot be maintained, and thus the above phenomena of shadows and cross-talk will occur. Therefore, it is a large technical concern to determine how to reduce power consumptions while making sure that these phenomena of shadows and cross-talk do not occur.
Note that if there are only four levels (V0 to V3) of multi-value supply voltages (a 4-level drive method), a configuration could be considered in which, for example, a p-type OP-amp (described later with reference to FIG. 8) is connected to a supply voltage V1 on the high-potential side and an n-type OP-amp (described later with reference to FIG. 10) is connected to a supply voltage V2 on the low-potential side (refer to Japanese Patent Publication No. Sho 62-53824). The reason for a configuration of this type is as follows. The p-type OP-amp is configured such that the input portion of its differential amplifier portion has an n-channel transistor and a constant-current source is connected to the low-potential power source side thereof. Therefore, in order to operate the n-channel transistor of the input portion and an n-channel transistor of the constant-current source normally (to make the voltage between the drain and source of each transistor sufficiently large), a high potential must be input to the transistor in the input portion, and that is why a p-type OP-amp is connected to V1. Conversely, the n-type OP-amp is configured such that the input portion of its differential amplifier portion has a p-channel transistor and a constant-current source is connected to the high-potential power source side thereof. Therefore, in order to operate the p-channel transistor of the input portion and a p-channel transistor of the constant-current source normally, a low potential must be input to the transistor in the input portion, and that is why an n-type OP-amp is connected to V2. This ensures that the operating voltage range of the OP-amps can be widened.
However, if there are five or more levels of multi-value supply voltages, and thus at least three impedance conversion means are necessary, it is not possible to determine what type of OP-amp should be used for the third and subsequent impedance conversion means, and thus it is a great technical problem to determine how the type should be determined.